The salient advantage of wireless telecommunications over wireline telecommunications is that the user of the wireless terminal is afforded the opportunity to use his or her terminal anywhere. On the other hand, the salient disadvantage of wireless telecommunications lies in that fact that because the user is mobile, an interested party might not be able to readily ascertain the location of the user.
Such interested parties might include both the user of the wireless terminal and a remote party. There are a variety of reasons why the user of a wireless terminal might be interested in knowing his or her location. For example, the user might be interested in telling a remote party where he or she is or, alternatively, the user might seek advice in navigation.
In addition, there are a variety of reasons why a remote party might be interested in knowing the location of the user. For example, the recipient of an E 9-1-1 emergency call from a wireless terminal might be interested in knowing the location of the wireless terminal so that emergency services vehicles can be dispatched to that location.
There are many techniques in the prior art for estimating the location of a wireless terminal. In accordance with some techniques, the location of a wireless terminal is estimated, at least in part, from signal measurements that are reported by the wireless terminal. The reported measurements are of signals measured by the wireless terminal that are transmitted by one or more base stations and, in some cases, by Global Positioning System (GPS) satellites. In order for these techniques to work, at least some of the transmitted signals have to be strong enough to allow for accurate measurement by the wireless terminal and for reliable processing by the particular estimation technique. Some of these techniques work well even in environments where the measured strengths of the different signals vary significantly, such as where signal obstructions are present, including natural obstructions such as mountains and artificial obstructions such as buildings.
There are also techniques in the prior art for estimating the elevation of a wireless terminal. Some of these techniques rely on the relationship between atmospheric pressure, PA, and elevation, ZA, in which PA decreases logarithmically with ZA, according to the formula:
                              Z          A                =                  H          ⁢                                          ⁢                      ln            ⁡                          (                                                P                  A                                                  P                  0                                            )                                                          (                  Eq          .                                          ⁢          1                )            wherein                P0 is the reference atmospheric pressure, and        H is the scale height of the atmosphere, which is the elevation at which the atmospheric pressure has decreased to e−1 times its value at mean sea level (e.g., approximately 7000 meters).        
It is well known in the prior art how to estimate the elevation of an object—such as an airplane—using Equation 1. Aircraft altimeters have used this technique for decades, and it is well known to be highly accurate. Furthermore, it is well known in the prior art how to estimate the elevation of a wireless terminal using Equation 1.
In order to obtain barometrically a good estimate of elevation (also known as a “z-estimate”), it is necessary to have both i) a good pressure measurement at the location for which the estimate of elevation is being determined and ii) a good pressure reference. The pressure reference can serve as an estimate of the sea-level pressure for a location of interest.